The invention relates to a method for improving the weldability of high-manganese-content, zinc coated steel strips which (in weight %) beside 6 to 30% manganese can contain up to 1% carbon, up to 15% aluminum, up to 6% silicone, up to 6.5% chromium, up to 4% copper and a content of titanium and zirconium in sum of up to 0.7% and a content of niobium and vanadium in sum of up to 0.5%, remainder iron and unavoidable steel tramp elements.
Due to their favorable elongation and strength properties, steels with high contents of manganese, aluminum and/or silicone, also known as HSD®-steels (High Strength and Ductility), are suited for the construction of transport vehicles, in particular for the automobile construction.
High-manganese-content steels (for example HSD®-steels) compared to conventional steels are characterized by a significantly lower specific weight, which contributes to a significant weight reduction when using these lightweight steels in the vehicle body construction.
Steels with high manganese content from 7 to 27 weight % are for example known from DE 199 00 199 A1 and steels with 9 to 30 weight % manganese are known from DE 10 2004 061 284 A1. Flat products made of these steels have high strengths and at the same time a high uniform elongation.
However, steels with higher manganese contents have a propensity of pitting and surface corrosion and without the addition of aluminum and/or silicone have a lower resistance against hydrogen-induced stress corrosion. It has therefore been proposed to also coat flat products made of high-manganese-content steels in a known manner with a metallic coating that protects the steel against corrosion.
When corresponding demands are placed on the corrosion resistance it is known to coat the high-manganese-content steel strip with an anticorrosion layer of zinc or a zinc alloy.
High-manganese-content steel strips may be in a cold rolled or hot rolled state, wherein the zinc or zinc alloy layer is generally applied eletrolytically or by hot dip galvanization.
From DE 199 00 199 A1 it is known to enrich and/or coat the surface of the flat product with aluminum. In WO 2007/075006 A1 a high-manganese-content steel is proposed, wherein the flat product produced from this steel in a known manner is coated electroytically or by hot dip galvanization with a zinc layer after the last final annealing.
An unsolved problem in many of the known high-manganese-content steels is still however that they are insufficiently weldable when galvanized, which is characterized by the occurrence of liquid metal induced crack formation in the welding zone of the material.
In the following, the term welding is understood to include all resistance-welding methods, fusion-welding methods or beam-welding methods in which beside the basic material the zinc coating is liquefied.
As a result of the heat influence during the welding, liquefied zinc from the coating infiltrates the grain boundaries. This infiltration causes the basic material to lose its strength and ductility in the vicinity of the welding zone to the degree that the welding connection or the basic material bordering the welding connection no longer meets the required mechanical demands and the risk of a premature failure of the welding connection due to crack formation increases. This form of stress corrosion is referred to as Liquid Metal Embrittlement (LME) because the corrosive medium in this case is a liquid metal.
For improving the weldability of hot dip galvanized high-manganese-content lightweight steels DE 10 2005 008 410 B3 purposes to apply an aluminum layer by means of PVD (Physical Vapor Deposition) to the cold strip prior to the final annealing. Onto this the anticorrosion coating is applied after the final annealing. The Al/Fe-diffusion layer formed as a result of the heat treatment is intended to prevent the entering of the molten zinc into the basic material during the welding and with this liquid metal embrittlement. However, a disadvantage is the high costs of this process.
For avoiding a liquid metal induced crack formation of high-manganese-content galvanized steels DE 10 2009 053 260 A1 proposes an annealing treatment in an N2/H2 containing atmosphere, which leads to a formation of aluminum nitrides on the steel surface. These surface-proximate aluminum layers are intended to prevent the entering of zinc into the material and with this embrittlement. A disadvantage is here the required high nitriding temperatures, which may lead to impaired mechanical properties of the basic material.